1. Field of the Invention
The present invention relates to an apparatus and method for performing process on a to-be-processed material, such as a semiconductor wafer, by a sheet-like plasma created through a reaction of a gas excited by an electron beam.
2. Description of the Related Art
In recent years, upon performing plasma process on a to-be-processed material, such as a semiconductor wafer, it has been necessary to perform a very fine process so that semiconductor devices can be manufactured as high-density units. For such very fine process to be achieved, it is necessary to set the directivity with which molecules reach the semiconductor wafer from a plasma created in a reaction chamber. For this reason, it is important to lower pressure in that reaction chamber in which the plasma is created, and to decrease a probability that the molecules in the reaction chamber will collide with each other.
If the pressure in the reaction chamber is simply reduced, then no plasma will be created due to a lowering in density of excited particles creating a plasma. A magnetron RIE method for accelerating the motion of electrons under a magnetic field so as to create a plasma even under a low pressure is disclosed, for example, in a document H. Okano and Y. Horiike: Jpn J. Appl. Phys. 21, 696 (1982) and an ECR method is disclosed in a document S. Matsuo and Y. Adachi: Jpn. J. Appl. Phys., 21, L4 (1982).
An Electron Beam Excited Plasma (EBEP) technique can generate a plasma even at a low pressure by continuously supplying electrons into a gas.
FIG. 1 diagrammatically shows one form of an apparatus for performing plasma process on a to-be-processed material, such as a semiconductor wafer, with the use of an EBEP technique as shown, for example, in the documents "T. Hara, M. Hamagaki, A, Sanda, Y, Aoyagi and S. Namba: J. Vac. Sci. Technol, 5, 366 (1987), J. Z. Yu, T. Hara, M. Hamagaki, T. Yoshinaga, Y. Aoyagi and S. Namba: J. Vac. Sci. technol, 6, 1626 (1988).
Such an apparatus for performing plasma process includes a chamber (hereinafter referred to as an electron source plasma creating chamber) 104 for creating a plasma, the chamber being located at one end side of a sealed container 100 and serving as an electron source. An electron beam acceleration chamber 112 is provided at an intermediate section of the sealed container 100. A reaction chamber 115 is provided on the other end side of the sealed container 10.
An inlet 101 for introducing an inert gas, such as Ar, electrodes 103, 105, 106 and 107 and electron beam-drawing annular coils 108, 109 and 110 are provided in the plasma creating chamber 104.
An annular coil 111 for guiding an electron beam 111, as well as an acceleration electrode 114 for accelerating the electron beam, is provided in the acceleration chamber 112.
A susceptor 116 is provided in the reaction chamber 115 to support a wafer 117 thereon and a gas inlet 118 is provided at the reaction chamber 115 to introduce a reaction gas therein.
A rectangular coil 201, solenoid coils 202, 203 and a samarium (Sm)-cobalt (Co)-based magnet 204 are arranged in proper places in the reaction chamber 115 as shown in FIG. 2.
Outlets 121, 122 and 123 are provided in the chambers 104, 112 and 115, respectively. The interiors of the respective chambers are evacuated by vacuum pumps to maintain these chambers at a required vaccum level.
An electrode 102 is connected to the electrode 103 to apply a required voltage Vd there. A switch 107a is connected to the electrode 105 to allow a selective connection to be made to a reference potential GND. A switch 107b is connected to the electrode 106 to allow a selective connection to be made to the reference potential GND. In the circuit as shown in FIG. 1, R1 to R4 designate resistors.
A power supply 113 is connected to the acceleration voltage 114 to apply a required acceleration voltage there. A power supply 119 is connected to the susceptor 116 to apply a required bias voltage Vt.
In the arrangement shown in FIG. 1, an electron source plasma is generated in the electron source plasma generation chamber 104. An electron beam is drawn from the electron source plasma and, being accelerated by the acceleration electrode 114, is introduced via the annular coil 111 into the reaction chamber 115.
In the reaction chamber 115, a gas introduced via the gas introducing inlet 118 is excited by an electron beam 120 to create a plasma through a reaction in the gas whereby it is possible to perform plasma process on a wafer 117.
In the above-mentioned apparatus thus constructed, however, if the electron beam thus introduced into the reaction chamber 115 is directly utilized, then the plasma becomes more dense along that area of the wafer where the electron beam is passed.
For this reason, if the apparatus is used in the vapor-phase growth (CVD) of a semiconductor device, in a dry-etching process, etc., a phenomenon occurs, such as the deposition speed and etching speed are fast at an area corresponding to the dense plasma area.
In order to prevent such phenomenon, research has been made into a method to uniformalize the density of a plasma. As one solution of this problem, permanent magnets 205a, 205b are employed with their N poles opposed to each other in a manner to define an electron beam path therebetween as shown in FIG. 3 whereby it is possible to set a sheet-like electron beam through a created magnetic field and hence to create a sheet-like plasma through the excitation of a gas by the electron beam in a reaction chamber. This method is disclosed, for example, in JPN PAT APPLN KOKAI PUBLICATION No. 59-27499.
If a semiconductor device is dry-etched through the utilization of such a sheet-like plasma, then the etching speed is made slower in a positive X-axis direction and faster in a negative X-axis direction as viewed in a direction of travel of the electron beam and an expected enhanced result cannot be obtained with respect to the uniformalization of etching.
Here, the X- and Y-axis represent a horizontal and a vertical direction relative to the wafer, respectively, and the Z-axis represents the direction in which the electron beam travels.
In order to investigate the causes for the different distribution of the etching speed in the positive X-axis and negative X-axis direction as set out above, simulation has been conducted on a large-scaled computer to see how electrons are mobilized under a magnetic field in the reaction chamber 115.
It has been found that the electron beam is configured to a sheet form through a mechanism as will be set out below, that is, on the wafer 117 supported on the susceptor 116 the sheet form of the electron beam 120 is tilted down in the positive X-axis direction and up in the negative X-axis direction, as shown in FIG. 4, as viewed in the Z-axis direction in which the electron beam travels.
FIGS. 5 and 6 are a front and a side diagrammatic view, respectively, where the electron beam is viewed in a direction of travel from the "inlet" side of the reaction chamber 115.
In the neighborhood of the inlet of the reaction chamber 115, that is, at that area as indicated by an arrow A in FIG. 6, a magnetic field is oriented by the annular coil 111 in the substantially Z-axis direction. In that area, the electrons are mobilized in the substantially Z-axis direction as in the case of the magnetic field and acquire those motion components, through collisions with other particles, other than the Z-axis component.
Given that the electrons acquire, for example, a motion component in the Y-axis direction, those electrons bearing a negative charge encounter an X-axis direction force under a magnetic field of a Z-direction and acquire a motion component of an X-axis direction.
FIG. 7 shows a relation among the direction of a magnetic field B, direction of motion of electrons e and sense of a force F which each electron receives.
When the electron acquires an X-axis direction motion component, then it similarly receives a negative Y-axis direction force this time. Consequently, through such a continued operation, the electron is spirally mobilized in the Z-axis direction, while being rotated around the Z-axis direction as shown in FIG. 8. That is, a cyclotron movement is conducted in a magnetic field.
Those magnets 205a and 205b are so arranged as to have their N poles opposed relative to each other with an electron beam path defined therebetween so that the senses of their neighboring lines of magnetic force are as indicated by arrows in FIG. 9.
When the electrons approach that area, since, of the electron motion including a rotation motion component, the Z-axis motion is still dominant, the electrons acquire forces in those directions of arrows in FIG. 10 by the lines of magnetic force as indicated by arrows in FIG. 9 and the electron beam is inclined in a positive X-axis up direction and a negative X-axis down direction as viewed in the Z-axis travel direction as shown in FIG. 11. In this case, this state is obtained in a range as indicated by an arrow B in FIG. 6.
Further, as shown in FIG. 4, the sheet-like plane of the electron beam over the wafer 117 is inclined, in a position past the centers of the magnets 205a and 205b, in a positive X-axis down and a negative X-axis up direction as viewed in the Z-axis direction in which the electron beam travels.
This is due to a reason as will be set out below.
In a position past the centers of the magnets 205a and 205b, a magnetic field is intensified in the Z-axis direction as shown in FIG. 12 and the electrons, moving in the Z-axis direction, are mobilized in a direction of arrows in an electron beam in FIG. 11.
The sheet-like electron beam acquires those forces as indicated by arrows in FIG. 13 and the electron beam is moved, as shown in FIG. 4, in the positive X-axis down and negative X-axis up direction as viewed in the Z-axis direction in which the electron beam travels. When the electrons are moved to some extent, they again start a spiral motion in the Z-axis direction.
Even in a position somewhat separate from the magnets, the electrons continue the Z-axis direction spiral motion, as they are, in a range as indicated by an arrow C in FIG. 6 and, consequently, over the wafer, the sheet-like plane of the electron beam stays inclined, as shown in FIG. 4, in the positive X-axis down and negative X-axis up direction as viewed in the Z-axis direction in which the electron beam travels. When, however, the sheet-like plane of the electron beam 120 set out above is inclined on a right lower side, the lower portion of the electron beam 120 as shown in FIG. 1 collides with a side surface of the susceptor 116 on which the wafer 117 is positioned. That portion of the electron beam fails to contribute to the creation of the plasma.
As a result, a plasma portion in the positive X-axis down direction is more weakened than a plasma portion in the negative X-axis up direction and the rate at which the plasma process is performed is fast in the negative X-axis direction, that is, on the left side of the wafer, and slow in the positive X-axis direction, that is, on the right side of the wafer.
In this way, the rate at which the plasma process is performed on the to-be-processed material is not uniform.
The conventional plasma process performing apparatus and method pose a problem, that is, the drawback that no uniform rate is obtained in performing plasma process on the material to be processed.